Optical chip and method for producing an optical chip having a reinforced structure

ABSTRACT

The invention relates to an optical conductor component in the form of an optical chip to which it is possible to connect optical conductors (LWL), such as, for example, individual fibres or fibre arrays and/or other optical conductor components such as photodetectors or photodetector arrays. The invention further relates to a method for producing such an optical chip.

[0001] The invention relates to an optical conductor component in the form of an optical chip to which it is possible to connect optical conductors (LWL), such as, for example, individual fibres or fibre arrays and/or other optical conductor components such as photodetectors or photodetector arrays. The invention further relates to a method for producing such an optical chip.

[0002] In order to be able to implement an optical coupling between planar LWL components, that is to say optical chips, and assemblies to be connected thereto, it is necessary for the optical chips to be strengthened structurally so that they withstand mechanical loads associated with the connection of optical conductors and other LWL assemblies. For this purpose, the chips are usually constructed in a thickened fashion by providing the entire underside of the respective chip with an additional glass plate which is pressed onto the chip after adhesive material has been interposed. The glass plate stiffens the chip structurally to a sufficient extent such that the latter withstands the mechanical loads to be expected.

[0003] Simply pressing the glass sheet onto the chip has, however, the disadvantage that the chip, which has a slightly curved shape because of its customary production process, experiences concave bending and bending back. The latter leads to strains in the chip, and thus to influences exerted on the optical conductors, made from glass, accommodated in the chip, since additional instances of refraction are caused in the optical conductors because of the strains. Furthermore, particularly in the case of optical chips with thermooptical structures, which are necessary, for example, for attenuating or for switching, the glass plate applied over the entire surface of the underside is disadvantageous in thereby worsening the good thermal conduction which is required in the reverse direction owing to the silicon or glass substrate of the chip.

[0004] U.S. Pat. No. 5,327,517 discloses a guided-wave circuit module with a guided-wave circuit chip having an input waveguide part, an output waveguide part and, positioned between, a guided-wave circuit; and a holder for supporting the chip, wherein the guided-wave circuit does not come in contact with the holder.

[0005] From EP 0 678 764 A1 a method for producing an integral optical device is known, wherein an optical waveguide structure is built on a silicon substrate, a reinforcing layer is formed above the waveguide structure and the region of the substrate underlying the waveguide structure is removed.

[0006] The invention creates an optical chip, with a thermooptical structure, which has adequate structural strength, and in the case of which there is lessening of the impairments, caused by strength-enhancing components, of its optical, in particular thermooptical properties. Furthermore, the invention creates a method for producing an optical chip with the aid of which the optical chip can be produced with improved optical properties, and yet with an adequate structural strength.

[0007] According to the invention, this is achieved by an optical chip with the features of claim 1 and by methods with the features according to claims 11 and 12, respectively. Preferred embodiments of the optical chip are described in claims 2 to 10.

[0008] The term “cohesive” stiffening structure is understood in this case to mean that the stiffening structure is not provided in a fashion consisting of separate individual elements, but that it extends continuously, that is to say without a break, on the substrate.

[0009] Because the thermooptical structures are left free from the stiffening structure, backward heat removal, that is to say downward heat removal, is not prevented below these structures. Consequently, it is possible to set up between the top and undersides of the chip an adequately steep temperature gradient which is essential for effective functioning of the thermooptical structures. An overall improvement in thermooptical properties of the chip is thereby achieved. Although provided as a layer, that is to say in a planar fashion, the stiffening structure has the function of a stiffening rib arrangement with the aid of which an adequate structural strength of the substrate is achieved. The strains in the substrate are reduced on those sections of the substrate which are left free from the stiffening structure, since the stiffening structure in these sections cannot force the substrate directly to adapt to its own shape. An improvement in the optical properties of the chip is achieved thereby. Optical couplings of LWL lines external to the chip and of assemblies can be arranged on the regions of the chip strengthened by the stiffening structure.

[0010] According to the embodiment according to claim 4, the stiffening structure is almost excluded from influencing the optical properties of the chip, because the substrate is not necessarily adapted to the shape of the stiffening structure, as a result of which strains would otherwise, as explained above, be produced in the substrate and the optical conductors arranged thereon. The explained stress-free connection between the substrate and stiffening structure is advantageous both in the case of optical structures provided as thermooptical structures, and in the case of the use of other optical structures such as AWG (Arrayed Waveguide Grating) or splitter structures. In the case of the use of non-thermooptical structures on the optical chip, the latter is likewise preferably provided with the stiffening structure not over the entire area but, in particular, with leaving free of the optical structures.

[0011] The production of a stress-free connection between substrate and stiffening structure is preferably performed as explained in claim 11.

[0012] It is to be noted in this case that the adhesive can also be applied in a layered fashion to the underside of the substrate by virtue of the fact that it is applied in a layered fashion to the side of the stiffening structure facing this underside, the stiffening structure then being laid onto the substrate.

[0013] Because the stiffening structure is merely laid onto the substrate, and the adhesive introduced between the substrate and the stiffening structure is cured simply without further external mechanical action on the substrate and the stiffening structure, a stress-free connection is achieved between substrate and stiffening structure which is attended by the abovenamed improvements with regard to the optical properties of the chip.

[0014] Consideration is given as adhesive to glues, in particular glues by means of which a satisfactory bonding of the stiffening structure on the substrate is achieved without excessive supply of heat.

[0015] The chip can be ground and polished at the circumference after the curing of the adhesive and the possible application of additional layers and/or assemblies to be applied to the top side of the substrate.

[0016] In the case when thermooptical structures are provided on the optical chip, according to the invention these structures are left free extensively from the stiffening structure, seen in the chip plane. In the case of other optical structures such as the abovenamed splitters and AWG structures, the stiffening structure can admittedly be applied to the entire lower lateral surface of the substrate of the optical chip, because there is no need to form a temperature gradient from top to bottom; however, it is preferably provided likewise only partially, in particular with leaving free of the optical structures, because the optical properties of the chip are thereby improved.

[0017] According to an embodiment of the invention, the stiffening structure is constructed with a central cutout in the form of a frame running partially around the edge of the substrate. The stiffening frame thereby achieved and arranged on the circumference of the chip results in a strong stiffening effect which permits the central cutout to be constructed with large dimensions in order for a plurality of different thermooptical structures to be arranged there. This stiffening structure is suitable, in particular, for rectangular chips, the thermooptical structures preferably being arranged along a longitudinal side of the chip.

[0018] According to another embodiment of the invention, the stiffening structure is constructed in the form of a cross which is arranged centrally on the substrate with the provision of diametrically opposite cutouts. This stiffening structure is suitable, in particular for circular chips. The bars of the cross of the stiffening structure preferably extend at least up to the lateral edge of the substrate such that optical lines such as, for example, glass fibres, can be led up to the chip from the side thereof and can be fitted there. The four cutouts produced are embraced thereby respectively on two sides of the stiffening structure in such a way that the circularly arcuate edge sides of the substrate, which are assigned to the cutouts, remain free.

[0019] In order to prevent strains of the substrate occurring owing to temperature fluctuations during operation, the stiffening structure is advantageously made from a material which has substantially the same coefficient of thermal expansion as the substrate material. Differences of the order of magnitude of 10⁻⁶ are to be understood in this case by “substantially”.

[0020] Glass is a suitable substrate material. Glass material functions adequately in stiffening and can be connected simply to the substrate, which, for its part, is made either from silicon, or likewise from glass, in particular by being glued. However, other materials with an adequate stiffening function and which can be handled well also come into consideration.

[0021] With the embodiment according to claim 7, the advantage is achieved that it is possible to form in the region of the thermooptical structure a steeper temperature gradient with temperature dropping from top to bottom through the chip substrate, the functioning of the thermooptical structure thereby being substantially improved. In this case, a protective structure constructed directly above the thermooptical structure, which is likewise preferably provided in the carrier layer, would lead to thermal conduction via direct contact with the carrier layer, such that heat would be conducted away upwards directly from the thermooptical structure, and this would lead to a reduction in the temperature immediately at the thermooptical structure, and thus to a reduction in the temperature gradient downwards. Given that, in accordance with the invention, the thermooptical structure is left free from the protective structure on the top side of the carrier layer, such a direct quick removal of heat does not take place, the heat transfer between the carrier layer and the air surrounding the latter being slight.

[0022] The stress-free connection between protective structure and carrier layer established according to claim 12 yields the same advantageous effects as in the case of the stress-free connection between the substrate and the stiffening structure provided on its underside.

[0023] It is preferred to provide both the protective structure and stiffening structure in combination.

[0024] The protective structure and the carrier layer are preferably made from a material which has essentially the same coefficient of expansion as the substrate material.

[0025] In accordance with the preferred material for the stiffening structure, glass is a preferred material for the protective structure. The protective structure need not be provided cohesively, but can be arranged split up into mutually separate parts only in those regions where grinding and polishing of the optical conductor structures, that is to say the optical conductor, is required in order to prevent them from being damaged, for example in the form of glass chippings. Consequently, the protective structure is provided directly below the optical conductor structures, at least at the edge sections of the chip, since external optical conductor fibres are connected at the edge of the chip to its optical conductor structures, the latter being processed by grinding and polishing the end face of the chip in order to construct optical connecting points on the edge side.

[0026] Preferred embodiments of the invention are explained below with reference to the drawing, in which:

[0027]FIG. 1 shows a schematic illustration of an optical chip according to an embodiment of the invention in a view from below,

[0028]FIG. 2 shows a schematic illustration of an optical chip according to another embodiment of the invention in a view from below, and

[0029]FIG. 3 shows a cross section through an optical chip according to an embodiment of the invention.

[0030] The optical chip 1 visible from FIG. 1 has a substrate 1′ (illustrated by the thick, rectangular line) in the form of a rectangle with different side lengths. The substrate 1′ is provided on its underside 2 with a stiffening structure 3 which extends along the two short edge sides 4, 5 and along one 6 of the two long edge sides (the top edge side in FIG. 1). The stiffening structure 3 thereby forms a U-shaped stiffening frame which runs round along three edge sides 4, 5 and 6, and thereby is a partially embracing frame. At the two longitudinal ends of the chip 1, the two limbs 7, 8 of the frame-shaped stiffening structure 3 thereby project a little, in the direction of the width of said chip, beyond the rectangular substrate 1′ thereof (illustrated by the thick line). According to this embodiment, the width of the frame-shaped stiffening structure 3 is just half as large as the width of the substrate 1′ of the chip 1. In the case when the chip 1 has larger dimensions or smaller dimensions as regards its length and width, the width of the frame-shaped stiffening structure can therefore be selected to be larger or smaller than the width of the substrate. Provided on the circumference of the chip 1 are four optical connecting points 9, 10, 11, 12 which are respectively arranged at points on the chip 1 where 

1. Optical chip (1, 100) having a substrate (1′, 100′), optical conductors (18) arranged on the top side of the substrate (1′, 100′), at least one thermooptical structure (14, 106, 107, 108, 109), arranged on the top side of the substrate (1′, 100′), for the purpose of influencing the optical properties of the optical conductors (18), and a cohesive, layered stiffening structure (3, 103) provided on the underside of the substrate (1′, 100′), the stiffening structure (3, 103) and the respective thermooptical structure (14, 106, 107, 108, 109) being provided in such a way that, seen in the top view of the chip (1, 100), the stiffening structure (3, 103) extends in two directions perpendicular to each other at a distance from the respective thermooptical structure (14, 106, 107, 108, 109) transversely over the substrate (1′, 100′) and/or along the free edge of the substrate (1′, 100′), at least partially around the respective thermooptical structure (14, 106, 107, 108, 109).
 2. Optical chip (1) according to claim 1, in which the stiffening structure (3) is constructed with a central cutout in the form of a frame running partially around the edge of the substrate (1′).
 3. Optical chip (100) according to claim 1, in which the stiffening structure (103) is constructed in the form of a cross which is arranged centrally on the substrate (100′) with the provision of diametrically opposite cutouts.
 4. Optical chip (1, 100) according to one of claims 1 to 3, the stiffening structure (3, 103) being connected to the substrate (1′, 100′) in a fashion free of stress.
 5. Optical chip (1, 100) according to one of claims 1 to 4, in which the stiffening structure (3, 103) is made from a material which has substantially the same coefficient of expansion as the substrate material.
 6. Optical chip (1, 100) according to one of claims 1 to 5, in which the stiffening structure (83, 103) is made from a glass material.
 7. Optical chip (1, 100) according to one of claims 1 to 6, in which the optical conductors (18) are provided in a carrier layer (17) arranged on the top side of the substrate (1′, 100′) and in which a layered protective structure (19) is formed on the top side of the carrier layer (17) and is provided over the optical conductors (18) at least at the edge sections of the optical chip (1, 100), and, seen in the top view of the chip (1, 100), extends at a distance from the respective thermooptical structure (14, 106, 107, 108, 109).
 8. Optical chip (1, 100) according to claim 7, in which the top-side protective structure (19) is connected to the carrier layer (17) in a fashion free of stress.
 9. Optical chip (1, 100) according to claim 7 or 8, in which the top-side protective structure (19) and the carrier layer (17) are made from a material which has essentially the same coefficient of expansion as the substrate material.
 10. Optical chip (1, 100) according to one of claims 7 to 9, in which the top-side protective structure (19) is a glass layer.
 11. Method for producing an optical chip (1, 100) according to one of claims 1 to 10, wherein a curable adhesive is applied in a layered fashion to the underside (2, 102) of the substrate (1′, 100′), the layered stiffening structure (3, 103) is laid onto the adhesive, and the adhesive is cured with the formation of a stress-free connection between the stiffening structure (3, 103) and the substrate (1′, 100′) without applying mechanical effects on the chip (1, 100).
 12. Method for producing a structurally strengthened optical chip (1, 100) according to one of claims 7 to 10, wherein a curable adhesive is applied in a layered fashion to the top side of the carrier layer (17), a layered protective structure (19) is laid onto the adhesive, and the adhesive is cured with the formation of a stress-free connection between the protective structure (19) and the carrier layer (17) without applying mechanical effects on the chip (1, 100). 